Details | Last modification | View Log | RSS feed
Rev | Author | Line No. | Line |
---|---|---|---|
6417 | ashmew2 | 1 | The Independent JPEG Group's JPEG software |
2 | ========================================== |
||
3 | |||
4 | README for release 6b of 27-Mar-1998 |
||
5 | ==================================== |
||
6 | |||
7 | This distribution contains the sixth public release of the Independent JPEG |
||
8 | Group's free JPEG software. You are welcome to redistribute this software and |
||
9 | to use it for any purpose, subject to the conditions under LEGAL ISSUES, below. |
||
10 | |||
11 | Serious users of this software (particularly those incorporating it into |
||
12 | larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to |
||
13 | our electronic mailing list. Mailing list members are notified of updates |
||
14 | and have a chance to participate in technical discussions, etc. |
||
15 | |||
16 | This software is the work of Tom Lane, Philip Gladstone, Jim Boucher, |
||
17 | Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, |
||
18 | Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG |
||
19 | Group. |
||
20 | |||
21 | IJG is not affiliated with the official ISO JPEG standards committee. |
||
22 | |||
23 | |||
24 | DOCUMENTATION ROADMAP |
||
25 | ===================== |
||
26 | |||
27 | This file contains the following sections: |
||
28 | |||
29 | OVERVIEW General description of JPEG and the IJG software. |
||
30 | LEGAL ISSUES Copyright, lack of warranty, terms of distribution. |
||
31 | REFERENCES Where to learn more about JPEG. |
||
32 | ARCHIVE LOCATIONS Where to find newer versions of this software. |
||
33 | RELATED SOFTWARE Other stuff you should get. |
||
34 | FILE FORMAT WARS Software *not* to get. |
||
35 | TO DO Plans for future IJG releases. |
||
36 | |||
37 | Other documentation files in the distribution are: |
||
38 | |||
39 | User documentation: |
||
40 | install.doc How to configure and install the IJG software. |
||
41 | usage.doc Usage instructions for cjpeg, djpeg, jpegtran, |
||
42 | rdjpgcom, and wrjpgcom. |
||
43 | *.1 Unix-style man pages for programs (same info as usage.doc). |
||
44 | wizard.doc Advanced usage instructions for JPEG wizards only. |
||
45 | change.log Version-to-version change highlights. |
||
46 | Programmer and internal documentation: |
||
47 | libjpeg.doc How to use the JPEG library in your own programs. |
||
48 | example.c Sample code for calling the JPEG library. |
||
49 | structure.doc Overview of the JPEG library's internal structure. |
||
50 | filelist.doc Road map of IJG files. |
||
51 | coderules.doc Coding style rules --- please read if you contribute code. |
||
52 | |||
53 | Please read at least the files install.doc and usage.doc. Useful information |
||
54 | can also be found in the JPEG FAQ (Frequently Asked Questions) article. See |
||
55 | ARCHIVE LOCATIONS below to find out where to obtain the FAQ article. |
||
56 | |||
57 | If you want to understand how the JPEG code works, we suggest reading one or |
||
58 | more of the REFERENCES, then looking at the documentation files (in roughly |
||
59 | the order listed) before diving into the code. |
||
60 | |||
61 | |||
62 | OVERVIEW |
||
63 | ======== |
||
64 | |||
65 | This package contains C software to implement JPEG image compression and |
||
66 | decompression. JPEG (pronounced "jay-peg") is a standardized compression |
||
67 | method for full-color and gray-scale images. JPEG is intended for compressing |
||
68 | "real-world" scenes; line drawings, cartoons and other non-realistic images |
||
69 | are not its strong suit. JPEG is lossy, meaning that the output image is not |
||
70 | exactly identical to the input image. Hence you must not use JPEG if you |
||
71 | have to have identical output bits. However, on typical photographic images, |
||
72 | very good compression levels can be obtained with no visible change, and |
||
73 | remarkably high compression levels are possible if you can tolerate a |
||
74 | low-quality image. For more details, see the references, or just experiment |
||
75 | with various compression settings. |
||
76 | |||
77 | This software implements JPEG baseline, extended-sequential, and progressive |
||
78 | compression processes. Provision is made for supporting all variants of these |
||
79 | processes, although some uncommon parameter settings aren't implemented yet. |
||
80 | For legal reasons, we are not distributing code for the arithmetic-coding |
||
81 | variants of JPEG; see LEGAL ISSUES. We have made no provision for supporting |
||
82 | the hierarchical or lossless processes defined in the standard. |
||
83 | |||
84 | We provide a set of library routines for reading and writing JPEG image files, |
||
85 | plus two sample applications "cjpeg" and "djpeg", which use the library to |
||
86 | perform conversion between JPEG and some other popular image file formats. |
||
87 | The library is intended to be reused in other applications. |
||
88 | |||
89 | In order to support file conversion and viewing software, we have included |
||
90 | considerable functionality beyond the bare JPEG coding/decoding capability; |
||
91 | for example, the color quantization modules are not strictly part of JPEG |
||
92 | decoding, but they are essential for output to colormapped file formats or |
||
93 | colormapped displays. These extra functions can be compiled out of the |
||
94 | library if not required for a particular application. We have also included |
||
95 | "jpegtran", a utility for lossless transcoding between different JPEG |
||
96 | processes, and "rdjpgcom" and "wrjpgcom", two simple applications for |
||
97 | inserting and extracting textual comments in JFIF files. |
||
98 | |||
99 | The emphasis in designing this software has been on achieving portability and |
||
100 | flexibility, while also making it fast enough to be useful. In particular, |
||
101 | the software is not intended to be read as a tutorial on JPEG. (See the |
||
102 | REFERENCES section for introductory material.) Rather, it is intended to |
||
103 | be reliable, portable, industrial-strength code. We do not claim to have |
||
104 | achieved that goal in every aspect of the software, but we strive for it. |
||
105 | |||
106 | We welcome the use of this software as a component of commercial products. |
||
107 | No royalty is required, but we do ask for an acknowledgement in product |
||
108 | documentation, as described under LEGAL ISSUES. |
||
109 | |||
110 | |||
111 | LEGAL ISSUES |
||
112 | ============ |
||
113 | |||
114 | In plain English: |
||
115 | |||
116 | 1. We don't promise that this software works. (But if you find any bugs, |
||
117 | please let us know!) |
||
118 | 2. You can use this software for whatever you want. You don't have to pay us. |
||
119 | 3. You may not pretend that you wrote this software. If you use it in a |
||
120 | program, you must acknowledge somewhere in your documentation that |
||
121 | you've used the IJG code. |
||
122 | |||
123 | In legalese: |
||
124 | |||
125 | The authors make NO WARRANTY or representation, either express or implied, |
||
126 | with respect to this software, its quality, accuracy, merchantability, or |
||
127 | fitness for a particular purpose. This software is provided "AS IS", and you, |
||
128 | its user, assume the entire risk as to its quality and accuracy. |
||
129 | |||
130 | This software is copyright (C) 1991-1998, Thomas G. Lane. |
||
131 | All Rights Reserved except as specified below. |
||
132 | |||
133 | Permission is hereby granted to use, copy, modify, and distribute this |
||
134 | software (or portions thereof) for any purpose, without fee, subject to these |
||
135 | conditions: |
||
136 | (1) If any part of the source code for this software is distributed, then this |
||
137 | README file must be included, with this copyright and no-warranty notice |
||
138 | unaltered; and any additions, deletions, or changes to the original files |
||
139 | must be clearly indicated in accompanying documentation. |
||
140 | (2) If only executable code is distributed, then the accompanying |
||
141 | documentation must state that "this software is based in part on the work of |
||
142 | the Independent JPEG Group". |
||
143 | (3) Permission for use of this software is granted only if the user accepts |
||
144 | full responsibility for any undesirable consequences; the authors accept |
||
145 | NO LIABILITY for damages of any kind. |
||
146 | |||
147 | These conditions apply to any software derived from or based on the IJG code, |
||
148 | not just to the unmodified library. If you use our work, you ought to |
||
149 | acknowledge us. |
||
150 | |||
151 | Permission is NOT granted for the use of any IJG author's name or company name |
||
152 | in advertising or publicity relating to this software or products derived from |
||
153 | it. This software may be referred to only as "the Independent JPEG Group's |
||
154 | software". |
||
155 | |||
156 | We specifically permit and encourage the use of this software as the basis of |
||
157 | commercial products, provided that all warranty or liability claims are |
||
158 | assumed by the product vendor. |
||
159 | |||
160 | |||
161 | ansi2knr.c is included in this distribution by permission of L. Peter Deutsch, |
||
162 | sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA. |
||
163 | ansi2knr.c is NOT covered by the above copyright and conditions, but instead |
||
164 | by the usual distribution terms of the Free Software Foundation; principally, |
||
165 | that you must include source code if you redistribute it. (See the file |
||
166 | ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part |
||
167 | of any program generated from the IJG code, this does not limit you more than |
||
168 | the foregoing paragraphs do. |
||
169 | |||
170 | The Unix configuration script "configure" was produced with GNU Autoconf. |
||
171 | It is copyright by the Free Software Foundation but is freely distributable. |
||
172 | The same holds for its supporting scripts (config.guess, config.sub, |
||
173 | ltconfig, ltmain.sh). Another support script, install-sh, is copyright |
||
174 | by M.I.T. but is also freely distributable. |
||
175 | |||
176 | It appears that the arithmetic coding option of the JPEG spec is covered by |
||
177 | patents owned by IBM, AT&T, and Mitsubishi. Hence arithmetic coding cannot |
||
178 | legally be used without obtaining one or more licenses. For this reason, |
||
179 | support for arithmetic coding has been removed from the free JPEG software. |
||
180 | (Since arithmetic coding provides only a marginal gain over the unpatented |
||
181 | Huffman mode, it is unlikely that very many implementations will support it.) |
||
182 | So far as we are aware, there are no patent restrictions on the remaining |
||
183 | code. |
||
184 | |||
185 | The IJG distribution formerly included code to read and write GIF files. |
||
186 | To avoid entanglement with the Unisys LZW patent, GIF reading support has |
||
187 | been removed altogether, and the GIF writer has been simplified to produce |
||
188 | "uncompressed GIFs". This technique does not use the LZW algorithm; the |
||
189 | resulting GIF files are larger than usual, but are readable by all standard |
||
190 | GIF decoders. |
||
191 | |||
192 | We are required to state that |
||
193 | "The Graphics Interchange Format(c) is the Copyright property of |
||
194 | CompuServe Incorporated. GIF(sm) is a Service Mark property of |
||
195 | CompuServe Incorporated." |
||
196 | |||
197 | |||
198 | REFERENCES |
||
199 | ========== |
||
200 | |||
201 | We highly recommend reading one or more of these references before trying to |
||
202 | understand the innards of the JPEG software. |
||
203 | |||
204 | The best short technical introduction to the JPEG compression algorithm is |
||
205 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
||
206 | Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. |
||
207 | (Adjacent articles in that issue discuss MPEG motion picture compression, |
||
208 | applications of JPEG, and related topics.) If you don't have the CACM issue |
||
209 | handy, a PostScript file containing a revised version of Wallace's article is |
||
210 | available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz. The file (actually |
||
211 | a preprint for an article that appeared in IEEE Trans. Consumer Electronics) |
||
212 | omits the sample images that appeared in CACM, but it includes corrections |
||
213 | and some added material. Note: the Wallace article is copyright ACM and IEEE, |
||
214 | and it may not be used for commercial purposes. |
||
215 | |||
216 | A somewhat less technical, more leisurely introduction to JPEG can be found in |
||
217 | "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by |
||
218 | M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides |
||
219 | good explanations and example C code for a multitude of compression methods |
||
220 | including JPEG. It is an excellent source if you are comfortable reading C |
||
221 | code but don't know much about data compression in general. The book's JPEG |
||
222 | sample code is far from industrial-strength, but when you are ready to look |
||
223 | at a full implementation, you've got one here... |
||
224 | |||
225 | The best full description of JPEG is the textbook "JPEG Still Image Data |
||
226 | Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published |
||
227 | by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95, 638 pp. |
||
228 | The book includes the complete text of the ISO JPEG standards (DIS 10918-1 |
||
229 | and draft DIS 10918-2). This is by far the most complete exposition of JPEG |
||
230 | in existence, and we highly recommend it. |
||
231 | |||
232 | The JPEG standard itself is not available electronically; you must order a |
||
233 | paper copy through ISO or ITU. (Unless you feel a need to own a certified |
||
234 | official copy, we recommend buying the Pennebaker and Mitchell book instead; |
||
235 | it's much cheaper and includes a great deal of useful explanatory material.) |
||
236 | In the USA, copies of the standard may be ordered from ANSI Sales at (212) |
||
237 | 642-4900, or from Global Engineering Documents at (800) 854-7179. (ANSI |
||
238 | doesn't take credit card orders, but Global does.) It's not cheap: as of |
||
239 | 1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7% |
||
240 | shipping/handling. The standard is divided into two parts, Part 1 being the |
||
241 | actual specification, while Part 2 covers compliance testing methods. Part 1 |
||
242 | is titled "Digital Compression and Coding of Continuous-tone Still Images, |
||
243 | Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS |
||
244 | 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of |
||
245 | Continuous-tone Still Images, Part 2: Compliance testing" and has document |
||
246 | numbers ISO/IEC IS 10918-2, ITU-T T.83. |
||
247 | |||
248 | Some extensions to the original JPEG standard are defined in JPEG Part 3, |
||
249 | a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84. IJG |
||
250 | currently does not support any Part 3 extensions. |
||
251 | |||
252 | The JPEG standard does not specify all details of an interchangeable file |
||
253 | format. For the omitted details we follow the "JFIF" conventions, revision |
||
254 | 1.02. A copy of the JFIF spec is available from: |
||
255 | Literature Department |
||
256 | C-Cube Microsystems, Inc. |
||
257 | 1778 McCarthy Blvd. |
||
258 | Milpitas, CA 95035 |
||
259 | phone (408) 944-6300, fax (408) 944-6314 |
||
260 | A PostScript version of this document is available by FTP at |
||
261 | ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz. There is also a plain text |
||
262 | version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing |
||
263 | the figures. |
||
264 | |||
265 | The TIFF 6.0 file format specification can be obtained by FTP from |
||
266 | ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme |
||
267 | found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems. |
||
268 | IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6). |
||
269 | Instead, we recommend the JPEG design proposed by TIFF Technical Note #2 |
||
270 | (Compression tag 7). Copies of this Note can be obtained from ftp.sgi.com or |
||
271 | from ftp://ftp.uu.net/graphics/jpeg/. It is expected that the next revision |
||
272 | of the TIFF spec will replace the 6.0 JPEG design with the Note's design. |
||
273 | Although IJG's own code does not support TIFF/JPEG, the free libtiff library |
||
274 | uses our library to implement TIFF/JPEG per the Note. libtiff is available |
||
275 | from ftp://ftp.sgi.com/graphics/tiff/. |
||
276 | |||
277 | |||
278 | ARCHIVE LOCATIONS |
||
279 | ================= |
||
280 | |||
281 | The "official" archive site for this software is ftp.uu.net (Internet |
||
282 | address 192.48.96.9). The most recent released version can always be found |
||
283 | there in directory graphics/jpeg. This particular version will be archived |
||
284 | as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz. If you don't have |
||
285 | direct Internet access, UUNET's archives are also available via UUCP; contact |
||
286 | help@uunet.uu.net for information on retrieving files that way. |
||
287 | |||
288 | Numerous Internet sites maintain copies of the UUNET files. However, only |
||
289 | ftp.uu.net is guaranteed to have the latest official version. |
||
290 | |||
291 | You can also obtain this software in DOS-compatible "zip" archive format from |
||
292 | the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or |
||
293 | on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12 |
||
294 | "JPEG Tools". Again, these versions may sometimes lag behind the ftp.uu.net |
||
295 | release. |
||
296 | |||
297 | The JPEG FAQ (Frequently Asked Questions) article is a useful source of |
||
298 | general information about JPEG. It is updated constantly and therefore is |
||
299 | not included in this distribution. The FAQ is posted every two weeks to |
||
300 | Usenet newsgroups comp.graphics.misc, news.answers, and other groups. |
||
301 | It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/ |
||
302 | and other news.answers archive sites, including the official news.answers |
||
303 | archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. |
||
304 | If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu |
||
305 | with body |
||
306 | send usenet/news.answers/jpeg-faq/part1 |
||
307 | send usenet/news.answers/jpeg-faq/part2 |
||
308 | |||
309 | |||
310 | RELATED SOFTWARE |
||
311 | ================ |
||
312 | |||
313 | Numerous viewing and image manipulation programs now support JPEG. (Quite a |
||
314 | few of them use this library to do so.) The JPEG FAQ described above lists |
||
315 | some of the more popular free and shareware viewers, and tells where to |
||
316 | obtain them on Internet. |
||
317 | |||
318 | If you are on a Unix machine, we highly recommend Jef Poskanzer's free |
||
319 | PBMPLUS software, which provides many useful operations on PPM-format image |
||
320 | files. In particular, it can convert PPM images to and from a wide range of |
||
321 | other formats, thus making cjpeg/djpeg considerably more useful. The latest |
||
322 | version is distributed by the NetPBM group, and is available from numerous |
||
323 | sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/. |
||
324 | Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is; |
||
325 | you are likely to have difficulty making it work on any non-Unix machine. |
||
326 | |||
327 | A different free JPEG implementation, written by the PVRG group at Stanford, |
||
328 | is available from ftp://havefun.stanford.edu/pub/jpeg/. This program |
||
329 | is designed for research and experimentation rather than production use; |
||
330 | it is slower, harder to use, and less portable than the IJG code, but it |
||
331 | is easier to read and modify. Also, the PVRG code supports lossless JPEG, |
||
332 | which we do not. (On the other hand, it doesn't do progressive JPEG.) |
||
333 | |||
334 | |||
335 | FILE FORMAT WARS |
||
336 | ================ |
||
337 | |||
338 | Some JPEG programs produce files that are not compatible with our library. |
||
339 | The root of the problem is that the ISO JPEG committee failed to specify a |
||
340 | concrete file format. Some vendors "filled in the blanks" on their own, |
||
341 | creating proprietary formats that no one else could read. (For example, none |
||
342 | of the early commercial JPEG implementations for the Macintosh were able to |
||
343 | exchange compressed files.) |
||
344 | |||
345 | The file format we have adopted is called JFIF (see REFERENCES). This format |
||
346 | has been agreed to by a number of major commercial JPEG vendors, and it has |
||
347 | become the de facto standard. JFIF is a minimal or "low end" representation. |
||
348 | We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF |
||
349 | Technical Note #2) for "high end" applications that need to record a lot of |
||
350 | additional data about an image. TIFF/JPEG is fairly new and not yet widely |
||
351 | supported, unfortunately. |
||
352 | |||
353 | The upcoming JPEG Part 3 standard defines a file format called SPIFF. |
||
354 | SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should |
||
355 | be able to read the most common variant of SPIFF. SPIFF has some technical |
||
356 | advantages over JFIF, but its major claim to fame is simply that it is an |
||
357 | official standard rather than an informal one. At this point it is unclear |
||
358 | whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto |
||
359 | standard. IJG intends to support SPIFF once the standard is frozen, but we |
||
360 | have not decided whether it should become our default output format or not. |
||
361 | (In any case, our decoder will remain capable of reading JFIF indefinitely.) |
||
362 | |||
363 | Various proprietary file formats incorporating JPEG compression also exist. |
||
364 | We have little or no sympathy for the existence of these formats. Indeed, |
||
365 | one of the original reasons for developing this free software was to help |
||
366 | force convergence on common, open format standards for JPEG files. Don't |
||
367 | use a proprietary file format! |
||
368 | |||
369 | |||
370 | TO DO |
||
371 | ===== |
||
372 | |||
373 | The major thrust for v7 will probably be improvement of visual quality. |
||
374 | The current method for scaling the quantization tables is known not to be |
||
375 | very good at low Q values. We also intend to investigate block boundary |
||
376 | smoothing, "poor man's variable quantization", and other means of improving |
||
377 | quality-vs-file-size performance without sacrificing compatibility. |
||
378 | |||
379 | In future versions, we are considering supporting some of the upcoming JPEG |
||
380 | Part 3 extensions --- principally, variable quantization and the SPIFF file |
||
381 | format. |
||
382 | |||
383 | As always, speeding things up is of great interest. |
||
384 | |||
385 | Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net. |